A system is known in which LAN (Local Area Network) signals that are standardized by IEEE802.3, for example, are transmitted and received using a wireless transmission line as a part of transmission lines. In such a system, a terminal station connected to a network device through a wired transmission line transmits LAN signals from the network device to a counterpart station, which is connected to the terminal station so as to be opposite each other through a wireless transmission line, and the counterpart station outputs the signals received via the wireless transmission line to the network device connected to the own station through a wired transmission line.
In contrast to the system described above, Patent Literature 1 discloses a technique in which if any of terminal stations connected through a wireless transmission line detects a fault in transmission lines (a wired transmission line and a wireless transmission line which are connected to the terminal station), the network device of the own station is notified of the fault, and the network device of the opposing terminal station is also notified of the fault.
In this technique, upon detection of a fault in a transmission line, the terminal station disconnects the wired transmission line connected to the terminal station. Further, the terminal station replaces a control signal sequence for notifying the fault with a data string, and outputs the data string to the wireless transmission line, thereby transmitting the control signal sequence to the opposing terminal station. Also when the above-mentioned control signal sequence is received from the opposing terminal station, the terminal station disconnects the wired transmission line of the own station.
According to this technique, when the terminal station detects a fault in a transmission line, the terminal station can notify the network device connected to the own station of the fault, and also when the opposing terminal station detects a fault in a transmission line, the terminal station can notify the network device connected to the own station of the fault.
With the recent advancement of IP technology, in the system as described above, each terminal station accommodates a plurality of wired transmission lines and has a switching function in many cases.
Patent Literature 1 discloses a technique in which each network device of the own station and each network device of the counterpart station are notified of a fault also in a system in which each terminal station accommodates a plurality of wired transmission lines (FIG. 8 and paragraphs [0076] and [0077] of Patent Literature 1).
In this technique, each terminal station has a function of detecting, for the plurality of wired transmission lines of the own station, a fault in the wired transmission lines, a function of detecting a fault in the wireless transmission line, and a function of outputting the above-mentioned control signal sequence to the wireless transmission line upon detection of a fault, and disconnects the wired transmission lines when a fault is detected and when the above-mentioned control signal sequence is received from the opposing terminal station.
According to this technique, also in the system in which each terminal station accommodates a plurality of wired transmission lines, when the own station detects a fault in a transmission line, the terminal station can notify the network device connected to the own station of the fault, and also when the opposing terminal station detects a fault in a transmission line, the terminal station can also notify the network device connected to the own station of the fault.
The present invention can be applied not only to a system using a wireless transmission line as a part of transmission lines, but also to a system in which transmission devices connected through a transmission line (corresponding to the above-mentioned wireless transmission line) that connects the transmission devices (corresponding to the above-mentioned terminal stations) transmit signals of a plurality of wired transmission lines connected to the transmission devices.
A research of related art was conducted after the present invention was made, and the research shows that Patent Literature 2 discloses a fault notification technique using a multiplexer in a system in which communication between frame relay terminals (hereinafter referred to as “FR terminals”), such as a frame relay router, is relayed through an ATM (asynchronous transmission mode) line.
The above-mentioned multiplexer includes a frame relay interface accommodation unit (hereinafter referred to as “RF interface accommodation unit”) which is provided for each RF terminal and accommodates the RF terminal; an ATM switch unit; and a plurality of ATM cell multiplexing units.
Frame data from each FR terminal is converted into an ATM cell by the RF interface accommodation unit that accommodates the FR terminal, and is then output to any of the plurality of ATM cell multiplexing units through the ATM switch unit.
Each of the ATM cell multiplexing units is configured to multiplex ATM cells from a plurality of outgoing lines of the ATM switch unit and to send out the multiplexed ATM cells to an ATM line. Each of the ATM cell multiplexing unit has a function of monitoring congestion. When congestion occurs, the ATM cell multiplexing unit needs to notify the FR terminal of the congestion, because the ATM cell received from the ATM switch unit is to be discarded.
Heretofore, when congestion is detected, the ATM cell multiplexing unit notifies, for each logical channel (virtual channel identifier: VCI) multiplexed, the RF interface accommodation unit corresponding to the logical channel of the congestion by using a management cell indicating the congestion. The RF interface accommodation unit having received the notification requests, for each notification, a frame logical channel (FR-DLCI) corresponding to the logical channel in the ATM of the notification. Further, the RF interface accommodation unit generates a notification frame which is in compliant with a frame relay standard interface protocol and in which a BENC bit is significant, and transmits the generated notification frame to the FR terminal.
According to the related art technique, however, when a plurality of logical channels is set between a single RF interface accommodation unit and the ATM cell multiplexing unit having detected the congestion, the congestion is notified for each logical channel. Accordingly, a plurality of congestion notifications is generated every time congestion is detected in a single ATM cell multiplexing unit. As a result, when the number of logical channels set to the same ATM line (the number of logical channels to be processed by a single ATM cell multiplexing unit) is large, the number of congestion notifications increases. Further, the RF interface accommodation unit performs processing of receiving the congestion notifications subsequently sent from the ATM cell multiplexing unit, and processing of transmitting the congestion notifications to the accommodated FR terminal, so that the congestion notification processing is complicated and the processing itself becomes congested. Furthermore, the ATM cell multiplexing unit also needs to execute the notification operation by the number of logical channels set to the same ATM line, which results in an increase in processing load.
Such problems occur also when the ATM cell multiplexing unit detects a fault in the ATM cell multiplexing unit itself or a fault in the ATM line and finally notifies the FR terminal of the fault.
According to the technique disclosed in Patent Literature 2, information on all logical channels connected to the ATM cell multiplexing unit is grouped and held for each ATM cell multiplexing unit. Further, when a monitor item abnormality (for example, the above-mentioned congestion or fault) is detected, monitor information including specific information on each group is notified to all RF interface accommodation units, the whole or a part of logical channels of which belong to the group, through the ATM switch unit. The RF interface accommodation units having received the notification, i.e., the RF interface accommodation units, the whole or a part of logical channels of which belong to the group, notify the RF terminal, which corresponds to the logical channel belonging to the group, of the monitor information. According to this technique, the ATM cell multiplexing unit notifies a single RF interface accommodation unit of the monitor information only once. This leads to a reduction in communication processing load of each functional block.